Technical Field of the Invention
The present invention relates to a projection zoom lens.
Description of the Related Art
A front-projection projector device configured to project a magnified image onto a screen in front of the device is recently widely used for presentations at companies, for education at schools and for home use.
As for a projection zoom lens, there has recently been a growing demand for “high magnification and wide angle”.
There have been known those described in Patent Documents 1 and 2 to meet the demand.
A projection zoom lens described in Patent Document 1 have a five-lens-group configuration of negative, negative, positive, negative and positive, and is capable of sufficiently suppressing aberrations during magnification change. However, a half field angle at a wide angle end: ωw remains at 30°.
A projection zoom lens described in Patent Document 2 have a five-lens-group configuration of negative, negative, positive, positive and positive, and is capable of sufficiently suppressing aberrations. However, a half field angle at a wide angle end: ωw remains at 20.8°.
A “lens used to project images” is generally different from a photographic lens system for camera in using “oblique rays” as imaging light.
In a photographic lens for camera, an entire effective region of the lens is usable.
However, in the lens used to project images, since a projection image is formed by oblique rays, a range usable as an image projection region is a part of the effective region of the lens.
For this reason, the projection zoom lens needs to have a wider angle to increase the area of a target projection surface on which images are projected.
Recently, there has also been a strong demand that a projection distance of the projector device be reduced and the projector device be “disposed closer” to the target projection surface.
There is a demand for a much wider angle in the projection zoom lens, in order to realize a large target projection surface with a configuration where the projection zoom lens is disposed close to the target projection surface as described above.
The present invention is made in view of the aforementioned circumstances. It is one object of the present invention to realize a novel projection zoom lens with a large field angle of a projection image.
From the viewpoint of applicability to various light bulbs, it is preferable that the projection zoom lens has a large back focus.
From the viewpoint of applicability to various light bulbs and wide angle, a ratio: Bf/Fw of a back focus: Bf to a focal length of the entire system at the wide angle end: Fw serves as an important parameter.
Looking at the projection zoom lens described in Patent Documents 1 and 2 in this light, the parameter “Bf/Fw” is about 0.12 in Patent Document 1 and about 0.69 in Patent Document 2.
It is one object of the present invention to realize a wide angle projection zoom lens capable of ensuring optical performance while setting a large “Bf/Fw” with a five-lens-group configuration in which the power arrangement of first to third lens groups is “negative, negative and positive”.
It has already been mentioned that there is a demand for a wider angle in the projection zoom lens, in order to realize a large target projection surface with the configuration where the projection zoom lens is disposed close to the target projection surface.
It is one object of the present invention to realize a projection zoom lens with a wide field angle.
Furthermore, in order to increase the area of a target projection surface on which images are projected, it is conceivable to achieve a wider angle of the projection zoom lens.
In contrast, recently, there has also been a strong demand that a projection distance of the projector device be reduced and the projector device be “disposed closer” to the target projection surface.
There is a demand for a much wider angle in the projection zoom lens, in order to realize a large target projection surface with a configuration where the projection zoom lens is disposed close to the target projection surface as described above.
In the projector as the image display device, an object of the projection zoom lens is an image display surface, and an imaging surface is a screen.
The image display surface and the screen are both flat. Therefore, good correction of “field curvature” is important in determining whether the projection zoom lens is good or bad.
Also, since an image to be projected is a magnified image, distortion in the projected image due to distortion is easily-noticeable. Therefore, good correction of the distortion is also important.
Furthermore, since the magnification is changed in the projection zoom lens, it is important that a variation in “field curvature and distortion” is small during the magnification change.
Therefore, It is one object of the present invention to realize a novel projection zoom lens with a wide field angle, which is capable of displaying a projection image with a large field angle.
It has already been mentioned that there have been known those described in Patent Documents 1 and 2 to meet such demand.
The projection zoom lens described in Patent Document 1 has a five-lens-group configuration of negative, negative, positive, negative and positive, and realizes a variable magnification ratio of approximately 1.5 times.
The projection zoom lens described in Patent Document 2 has a five-lens-group configuration of negative, negative, positive, positive and positive, and realizes a variable magnification ratio of approximately 1.7 times.
In the projection zoom lens described in Patent Document 1, first, third and fifth lens groups are fixed groups, and magnification change from a wide angle end to a telephoto end is performed by moving second and fourth lens groups toward the reduction side.
In the projection zoom lens described in Patent Document 2, first and fifth lens groups are fixed groups, and magnification change from a wide angle end to a telephoto end is performed by moving second to fourth lens groups toward the reduction side.
In the projection zoom lenses described in Patent Documents 1 and 2, the first lens group on the magnification side and the fifth lens group on the reduction side are the fixed groups.
Therefore, the moved groups need to be displaced in a “displacement area with both ends fixed”.
Generally, a displacement amount of the moved groups needs to be increased to increase the variable magnification ratio. A reduction in the displacement area of the moved groups results in an increase in a rate of change in the magnification. As a result, aberrations are likely to be deteriorated.
When the moved groups are increased in size with the number of lens groups such as five, the entire lens system is likely to be increased in weight and length.
The present invention is made in view of the aforementioned circumstances. It is one object of the present invention to realize a projection zoom lens having a five-lens-group configuration and employing a novel magnification varying method, thereby achieving a high variable magnification ratio.
It has already been mentioned that there is a demand for a much wider angle in the projection zoom lens, in order to realize a large target projection surface with the configuration where the projection zoom lens is disposed close to the target projection surface.
From the practical aspect 36, it is preferable that the projection zoom lens is compact and light in addition to having a wide angle.
Among the lens groups included in the projection zoom lens, the one most likely to grow in size and weight is the first lens group.
When the first lens group is heavy, not only the projection zoom lens itself becomes heavy but also “eccentricity caused by its own weight” may occur in the first lens group.
From this viewpoint, in both of the projection zoom lenses described in Patent Documents 1 and 2, the first lens group has a four-lens configuration. Thus, the projection zoom lenses still have room for improvement in terms of reduction in weight of the first lens group.
The present invention is made in view of the aforementioned circumstances.
More specifically, it is one object of the present invention to realize a projection zoom lens which has a wide field angle and is capable of reduction in size and weight.
It has already been mentioned that there is a demand for a much wider angle in the projection zoom lens, in order to realize a large target projection surface with a configuration where the projection zoom lens is disposed close to the target projection surface.
The present invention relates to the combination of the refractive powers of the constituent lenses. It is one object of the present invention to realize a projection zoom lens of a five-lens-group type, which achieves high performance across the entire zoom area.